This invention relates to high resolution intravascular imaging and more particularly to ultrasound imaging and techniques for enhancing image quality.
In ultrasound imaging, echoes from blood molecules degrade the lumen-to-vessel wall contrast. However, the production of high resolution images of vessel wall structures requires imaging at high ultrasound frequencies. The back scatter from blood in such an image is a significant problem in high frequency ultrasound imaging, since the scattering of ultrasound from blood is proportional to the fourth power of the frequency such that the higher the ultrasound frequency the more pronounced is the back scatter from blood.
It is desirable to provide imaging over a broad range of frequencies. Therefore, echoes in the ultrasound image due to blood cells must either be removed or suppressed to a level at which wall structures can be distinguished from blood. Since blood is typically in motion relative to the image of interest, an individual frame will contain speckle due to the interference of blood constituents. One technique for enhancing image quality is to average successive image frames thereby to smooth out the impact of speckle (that is, the irregular pattern of backscatter from blood cells) in the ultrasound images. Thresholding-based techniques have been suggested wherein blood-speckle induced smearing is removed based on a signal intensity compared with a threshold. However, such techniques are not effective in reducing the mean echo amplitude from the region of blood flow, and they cannot totally remove blood echoes from the image. What is needed is a mechanism for identifying the signatures of blood-induced echoes and for subsequently suppression which allows a display of ultrasound images free of blood-induced echoes.
Background information on the subject of intravascular ultrasonography is found in U.S. Pat. No. 4,794,931 and related U.S. Pat. No. 5,000,185.